Diphenylphenoxy-Thiophene-PDI Dimers as Acceptors for OPV Applications with Open Circuit Voltage Approaching 1 Volt.

Caterina Stenta,Lluis Marsal,Aurélien Viterisi,Desiré Molina,Fernando Fernández-Lázaro,Werther Cambarau,Ángela Sastre-Santos,Emilio Palomares,Sara Pla,María Montero-Rama

doi:10.3390/nano8040211

Abstract

Two new perylenediimides (PDIs) have been developed for use as electron acceptors in solution-processed bulk heterojunction solar cells. The compounds were designed to exhibit maximal solubility in organic solvents, and reduced aggregation in the solid state. In order to achieve this, diphenylphenoxy groups were used to functionalize a monomeric PDI core, and two PDI dimers were bridged with either one or two thiophene units. In photovoltaic devices prepared using PDI dimers and a monomer in conjunction with PTB7, it was found that the formation of crystalline domains in either the acceptor or donor was completely suppressed. Atomic force microscopy, X-ray diffraction, charge carrier mobility measurements and recombination kinetics studies all suggest that the lack of crystallinity in the active layer induces a significant drop in electron mobility. Significant surface recombination losses associated with a lack of segregation in the material were also identified as a significant loss mechanism. Finally, the monomeric PDI was found to have sub-optimum LUMO energy matching the cathode contact, thus limiting charge carrier extraction. Despite these setbacks, all PDIs produced high open circuit voltages, reaching almost 1 V in one particular case.

Highlights

The power conversion efficiency (PCE) of organic solar cells (OSCs) has surged in the last decade
Atomic force microscopy, X-ray diffraction, charge carrier mobility measurements and recombination kinetics studies all suggest that the lack of crystallinity in the active layer induces a significant drop in electron mobility
Bragg–Brentano diffractograms and grazing incidence X-ray diffraction (GIXRD) 2D area images of active layers made from each PDI were recorded in identical conditions as those previously described for PTB7/PCBM blends [43,44]

Summary

Introduction

The power conversion efficiency (PCE) of organic solar cells (OSCs) has surged in the last decade. Those made from bulk heterojunction solution-processed active layers have shown the most potential for practical applications [1] In this respect, polymer and small molecule donors have both been the focus of significant research, leading to PCEs of over 10% when blends containing a fullerene-based acceptor [2,3,4,5] perylenediimide are used. The perpendicular arrangement of the diphenylphenoxy group with respect to the perylene core was shown to effectively reduce the aggregation of the perylene derivatives in solution [41,42] We extended this concept to the solid state by applying diphenylphenoxy-substituted PDIs to OSCs. We carried out the synthesis and characterization of two new PDI dimers bridged through the bay position with one and two thiophene units and applied them to bulk-heterojunction solar cells. The optical and electronic properties of the active layers of all the PDIs were assessed via a study of non-geminate recombination kinetics, using the charge extraction/transient photovoltage (CE/TPV) method and electron and hole mobility measurements

Synthesis and Characterization of the PDI-Acceptors

Solar Cell Devices Fabrication and Characterization

Morphologival Characterization

Electrical and Photophysical Characterization

Conclusions